1. Field of the Invention
The present invention generally relates to lens units and projection screens made of the same, and more particularly to a lens unit with a scattering layer comprising a transparent resin blended with scattering particles and a projection screen including the lens unit.
2. Description of Related Art
Currently, projectors and projection screens have been widely used in teaching, meeting presentation, advertising, entertainment and so on. Although the projectors and projection screens are improved with advanced technology, some drawbacks exist in the front projection technology used in a projector or projection screen. For example, when using the projector or projection screen, the ambient light from the ambient environment of the projector will adversely affect the projection. When the ambient light becomes stronger, the contrast of projection images on the projection screen becomes worse. As a result, it becomes difficult to clearly see the content of the projection images. Therefore, it is necessary to eliminate the ambient light, which is normally hard eliminated. Nevertheless in usual circumstances, such as in teaching or meeting presentations, complete elimination of the ambient light will cause inconvenience to the audiences who may need to make some notes.
On the other hand, along with the rapid development of high luminance white light LEDs and RGB laser diodes, the projectors are developing towards miniaturization and portability. However, miniaturized projectors lack high luminance light sources. Therefore, poor contrast of projection images on the projection screen caused by the ambient light becomes much more serious.
To overcome the above-described drawback of the ambient light causing the poor contrast, the projection screens in the prior art were all designed to selectively absorb the ambient light and reflect the light from the projector light source due to difference between the light from the projector light source and the ambient light.
In order to maximally reflect the light from the projector light source and reject the ambient light so as to increase the contrast of image, the following methods are proposed in the prior arts:                (1) Angle selection: since the incident angle of the light from the projector light source and the ambient light are different, the angle selection can be made through light directivity.        (2) Polarization selection: when the ambient light is an unpolarized light and the light from the projector light source is designed as a polarized light, the polarization selection can be achieved.        (3) Wavelength selection: since the projector light source is an RGB light source while the ambient light has a broadband spectrum, the wavelength selection can be achieved.        
The angle selection method for improving the contrast between the light from the projector light source and the ambient light and increasing the viewing angle is disclosed in U.S. Pat. Nos. 1,942,841, 4,298,246, 5,096,278 and Taiwan Patent No. 293887.
Taiwan Patent No. 293887 discloses a method of increasing the screen contrast by adjusting spacing of retro-reflective material areas using a self-aligning method. However, the method still has some drawbacks to be solved.
FIG. 1A shows a lens unit as disclosed in Taiwan Patent No. 293887. The lens unit includes a micro lens 20, a light absorbing layer 21 and a reflective layer 23.
The micro lens 20 has a light incident surface 201 and a light emergent surface 202 opposing to the light incident surface 201. The light absorbing layer 21 is formed on the light emergent surface 202 of the micro lens 20 and has a cavity 211 filled with a transparent material or white material, for example TiO2, such that when the light from the projector light source enters into the micro lens 20 and reaches the reflective layer 23, a pure reflective surface can be formed by the transparent material in the cavity 211 or a Lambertian surface can be formed by the white material in the cavity 211 for light reflection.
As shown in FIG. 1B, the cavity 211 of the lens unit is filled with a transparent material so as to form a pure reflective surface 22a, for example a mirror surface, for light reflection. When incident lights pass through the light incident surface 201 of the micro lens 20 and leave the light emergent surface 202 of the micro lens 20, and then reach the light absorbing layer 21 or the mirror surface 22a, the incident ambient light from the ambient environment of the projector in a direction S2 is absorbed by the light absorbing layer 21 and the incident light from the projector light source in a direction S1 reaches the mirror surface 22a and is reflected by the mirror surface 22a based on the rule that the angle of incidence is equal to the angle of reflection.
Therefore, the lens unit using the mirror surface 22a improves the light energy utilization efficiency, while such lens limits the viewing angle at the same time.
As shown in FIG. 1C, the cavity 211 of the lens unit is filled with a white material such as white photoresist so as to form a Lambertian surface 22b. When incident lights pass through the light incident surface 201 of the micro lens 20 and leave the light emergent surface 202 of the micro lens 20, and then reach the light absorbing layer 21 or the Lambertian surface 22b, the incident ambient light from the ambient environment of the projector in a direction S2 is absorbed by the light absorbing layer 21 and the incident light from the projector light source in a direction S1 is uniformly diffused by the Lambertian surface 22b. As shown in FIG. 2, the Lambertian surface 22b uniformly diffuses the incident light. However, light with a relative large angle (θ>θc) will be constrained inside the lens unit due to the total internal reflection, and part of the constrained light will be reflected several times by the light incident surface 201 of the micro lens 20 and finally absorbed by the light absorbing layer 21, thereby reducing the whole luminance.
Therefore, the lens unit with the Lambertian surface 22b increases the viewing angle but decreases the light energy utilization efficiency.
Accordingly, there is a need to provide a lens unit and a projection screen made of the same so as to increase the light energy utilization efficiency and increase the viewing angle.